1. Field of the Invention
The present invention concerns a method and x-ray apparatus to determine the effective skin input dose in x-ray examinations. In particular, it concerns a method and apparatus of the type wherein the skin input dose is obtained by dividing the measured dose area product by the exposed skin input area, which is calculated from the exposed area in the film plane or image intensifier input screen plane, geometric data of the x-ray system, and the thickness of the body part of the patient to be examined.
2. Description of the Prior Art
Conventional x-ray systems possess a dose area measurement device that generally is disposed beneath the radiation diaphragm in the beam path. This measurement device has a measurement chamber and supplies the dose area product, which is also abbreviated as a dose area. More important than the dose area product, however, is the effective operative dose to which the patient is exposed in the x-ray examination. This effective operative dose or skin input dose is calculated from the dose area product divided by the effective exposed skin input area. The skin input area is different from patient to patient; it depends in particular on the thickness of the body part of the patient to be examined, since the distance of the focus from the skin surface is different in each patient. The diameter or the thickness of the body part to be examined can fluctuate dependent on the body position and build, for example between 0 cm and 50 cm. Because of such variations, in many cases only the dose area product is registered, but this is not very significant.
It is also possible to measure the distance of the radiation source to the patient manually with a measuring tape; but this procedure is very difficult and not particularly precise.
To determine and adjust a desired radiation dose in x-ray apparatuses, it is specified in German OS 197 30 519 to firmly mount a dosimeter in the beam path of the x-ray tube to measure the radiation dose emitted by the x-ray tube. Thus the radiation dose thus is determined and not the skin input dose.
German Patent 38 10 501 C2 discloses an x-ray measurement apparatus with two radiation detectors disposed in the primary beam cone, between which lies a region of an examination subject. From output signals of these detectors a signal corresponding to the patient transparency is formed. A range finder for the focus-patient distance (FOBJ) is present, and the incident dose power given irradiation, exposure without a patient, as well as the patient transparency given exposure, are determined from the output signals of the radiation detectors and the range finder. Again, the skin input dose is not determined here.